It is well established in the prior art that absorbable fibers suitable for constructing biomedical constructs with prolonged strength retention profile, as in certain surgical sutures and meshes as well as prosthetic tendons and ligaments, need to be based on polymers having (1) high molecular weight; (2) a high degree of crystallinity; and (3) minimum or no monomeric species. These requirements were claimed to have been fulfilled by the l-lactide/glycolide copolymers described in U.S. Pat. No. 5,425,984 and EP Application No. 241,252 (1987). However, in certain high load-bearing applications where a prosthetic fibrous construct experiences cyclic stresses and is expected to maintain a substantial fraction of its initial strength for several weeks post-operatively, additional requirements are imposed. Typical examples of such constructs are surgical meshes for hernia repair and prosthetic tendons and ligaments. These additional requirements are expected to be associated with having a high degree of toughness, as measured in terms of the work required to break, without compromising, significantly, their high tensile strength, high elastic modulus, low stretchability, and high yield strength. Such requirements also are expected to be associated with a polymeric chain with higher hydrolytic stability than those containing glycolate sequences are. Unfortunately, the prior art of absorbable polymers provides conflicting teachings that may be applied towards meeting the aforementioned additional requirements. To increase toughness, the introduction of more flexible 6-caprolactone-based sequences in polyglycolide chain has been used successfully in the production of low modulus sutures (see, for example, U.S. Pat. Nos. 4,605,730 and 4,700,704) but with compromised strength. A similar situation is encountered in the copolymer of glycolide and trimethylene carbonate (see, for example. U.S. Pat. No. 4,429,980). Interestingly, fibers made of these two types of copolymers do display a lower propensity to hydrolysis than polyglycolide, but their strength loss profiles remain unsuitable for long-term, load-bearing applications. Unexpectedly, the present invention describes a copolymeric xcex5-caprolactone and trimethylene carbonate based compositions, which meet the above noted stringent requirement for fibers suited for the construction of biomedical devices that are expected to (1) support high loads; (2) experience cyclic stresses; (3) display minimum stretchability; (4) display a high degree of toughness; (5) display optimum hydrolytic stability; and (6) possess a prolonged strength profile, particularly during the initial post-operative period.
The present invention is directed to a crystalline copolymer which is a copolymer of l-lactide and at least one cyclic monomer which is a liquid at or above about 40xc2x0 C., wherein the l-lactide derived sequences of the polymer chain comprise from about 86 to about 99 percent of all sequences, and wherein the copolymer has a Tm of at least 150xc2x0 C., exhibits a crystallinity of at least about 25%, and has an inherent viscosity of at least about 1.4 dl/g.
Preferably the cyclic monomer is xcex5-caprolactone, trimethylene carbonate, or both. Molar ratios of l-lactide to cyclic monomer which are within the scope of the present invention include 86 to 14, 87 to 13, 88 to 12, 89 to 11, 90 to 10, 91 to 9, 92 to 8, 93 to 7, 94 to 6, 95 to 5,96 to 4,97 to 3,98 to 2, and 99 to 1.
A monofilament suture made from the copolymer of the present invention has an elastic modulus of greater than about 400,000 psi, a tensile strength of greater than about 40,000 psi, and a percent elongation of less than about 50%.
Also within the scope of the present invention are multifilament yarns made from the copolymer of the present invention. Such multifilament yarns which may be employed as surgical sutures or may be formed into a surgical device or construct such as, for example, a mesh, a prosthetic tendon, a ligament or a vascular graft.
The present invention is directed to high molecular weight copolymers of a major portion of l-lactide and a minor portion of xcex5-caprolactone (CL) or trimethylene carbonate (TMC) or both. High molecular weight is defined as displaying an inherent viscosity of at least 1.4 dl/g. The molar ratio of l-lactide to comonomer is between from about 86 to about 14 and from about 99 to about 1. Preferably, the ratio is in the range of from about 91 to about 9, more preferably from about 92 to about 8 and most preferably from about 95 to about 5. The present copolymers, particularly the l-lactide/caprolactone copolymers of the present invention, have a degree of crystallinity of greater than about 25%.
The present invention is also directed to l-lactide/caprolactone, l-lactide/trimethylene carbonate and l-lactide/caprolactone/trimethylene carbonate-based monofilament yarn having a Young""s modulus of more than 400,000 psi, a tensile strength exceeding 40,000 psi, a percent elongation of less than 50%, a Tm of greater than about 150xc2x0 C., and a degree of crystallinity exceeding 25%. The present invention is also directed to multifilament yarn having a tenacity in excess of 3 g/d with single fiber diameter of less than 35xcexc. In accordance with the present invention, surgical suture made of such monofilament and multifilament yarns absorb in less than 3 years and maintain at least 50% of their initial strength at three weeks post-operatively, preferably at six weeks post-operatively. Also within the scope of the present invention are prosthetic ligaments, tendons, meshes for tissue repair, and vascular grafts made totally of such multifilament yarns or a combination the present multifilaments and monofilaments or a combination with other more absorbable multifilament or monofilament yarns.